1. Field of Invention
The present invention relates to protection circuits. More particularly, the present invention relates to the protection circuit of an integrated circuit.
2. Background of the Invention
An over-voltage protection circuit is typically used in an integrated circuit to protect the integrated circuit from lightning surge and ESD (electric static discharge). Referring to FIG. 1, an over-voltage crowbar connects with a protected circuit in parallel. When the surge voltage is introduced into the circuit, the over-voltage crowbar clamps the voltage under the absolute maximum value to avoid the circuits from over-voltage breakdown. FIG. 2 shows a traditional crowbar circuit. If the surge voltage in the Vcc is higher than the zener voltage (Vz) of a zener diode 31, a clamping transistor 35 is driven to clamp the voltage in Vcc. Since the energy of the surge voltage in the Vcc is reduced instantly by the clamping transistor 35, a fast response is needed. Furthermore, a low Rds-on (turn-on impedance from drain to source) of the clamping transistor 35 is required. The Rds-on can be calculated as follows:
Ids=Kxc3x97[(Vgs Vt)xc3x97Vd(V2ds/2)]K="sgr"xc3x97(W/L)Rds-on=Vds/Ids=L/{Wxc3x97"sgr"xc3x97[(Vgsxe2x88x92Vt)xe2x88x92(Vds/2)]}xe2x80x83xe2x80x83(1)
In the above equations, Vds is a drain-to-source voltage, Vgs is a gate-to-source voltage, Vt is a threshold voltage, "sgr" is the product of mobility and oxide capacitance/unit, W is the width of a transistor, and L is the length of the transistor.
According to Equation (1), we find that the Rds-on is proportional to the Vds and inversely proportional to the Vgs. In order to gain a low Rds-on, the clamping transistor 35 needs a higher Vgs when Vds is high. Unfortunately, the surge voltage in the Vcc creates a high Vds for the clamping transistor 35 and a higher Rds-on. In addition, an existing negative feedback phenomenon restricts the response time and impedance of the Rds-on. Referring to the Equation 1), Vgs is correlated to the Vds, where Vgs=Vdsxe2x88x92Vz. Therefore, even when a bigger size of the clamping transistor 35 and the Zener diode 31 are used in FIG. 2, it is still hard to obtain a fast response and the low impedance performance.
This invention provides an over-voltage crowbar for lightning surge and ESD protection. The over-voltage crowbar comprises a Zener diode, a mirror amplifier (mirror-amp), two resistors, a speed-up capacitor and a clamping transistor.
Once the surge voltage in Vcc is higher than the Zener voltage, the mirror-amp is driven to generate an amplified voltage. The amplified voltage works together with the speed-up capacitor that easily turns on the clamping transistor. Two resistors are used for grounding the input of the mirror-amp and the input of the clamping transistor respectively to ensure that the mirror-amp and the clamping transistor are off in the normal condition. The mirror-amp includes an n-transistor and two p-transistors, which provide sufficient headroom to drive the clamping transistor on.
Advantageously, the over-voltage crowbar of this invention rapidly drives the clamping transistor to low impedance; thereby achieving improved sustenance rating for lightning surge and ESD in the integrated circuit.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.